use Compress::Zlib;

my $gunzip = inflateInit(WindowBits => 16 + MAX_WBITS)
       or die "Cannot create a inflation stream\n" ;
...
    $mech->add_handler( response_data => sub {
      my ( $response, $ua, $h, $data ) = @_;
      my ($buffer, $status) = $gunzip->inflate($data);
      # uncompressed data in $buffer

      # return true to get called again for same response.
      1; 
    } ;